A sheet carrying device is provided in an image forming apparatus, for example, a digital copying machine. The sheet carrying device serves as an automatic document carrying device in a case where a document is carried, and serves as a paper feeding device including a paper feeding cassette in a case where printing paper is carried.
In such a sheet carrying device, a sheet is picked up one sheet at a time by a pickup roller, from a tray on which sheets such as documents or paper are mounted. Thus picked-up sheet is carried by use of a plurality of pairs of carrying rollers.
Here, the sheet carrying device is required to appropriately carry various types of papers as a result of (i) diversification of document paper types and (ii) use of printing paper suitable for color photocopying, so that no malfunction such as jamming of paper occurs in paper carrying. Moreover, image forming apparatuses are required to accelerate a speed of a printing process. High-speed paper carrying is essential for satisfying this requirement.
The sheet carrying device has the following problem in satisfying the aforementioned requirements. For example, when various sheets are to be carried, a frictional force between sheets or between the sheet and the pickup roller is not constant, depending on a type of each sheet. Usually, the frictional force differs between the types of sheets. Moreover, the pickup roller deteriorates over time, and is contaminated by, for example, adhesion of paper powder and/or oil. Due to these circumstances, the pickup roller may slip on the sheet when the sheet is picked up from the tray. Slipping on the sheet, the pickup roller cannot carry the sheet to a predetermined carrying destination within a predetermined time. Therefore, a high speed process becomes difficult in a printing process in the image forming apparatus.
On the other hand, when the slip occurs, the pickup roller retries a paper feeding operation. However, under the condition in which the slip of the pickup roller has occurred, there are cases in which it is difficult to feed the sheet by a simple retry.
In view of the above problem, in Patent Document 1, when a slip has occurred, a pickup roller retries a paper feeding operation at a decreased rotation speed (decreased maximum speed) of the pickup roller.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 269256/2004 (Tokukai 2004-269256) (published on Sep. 30, 2004)
[Non-Patent Document 1]
Yoshimasa Tsuruoka, Yasushi Toyoda, and Yoichi Hori, “Basic Study on Traction Control of Electric Vehicle”, Transactions of Institute of Electrical Engineers of Japan: D, Vol. 118-D, No. 1, pp 45-50, 1998.1.
However, in the arrangement of Patent Document 1, although feeding of the sheet becomes possible by carrying out the retry at a decreased rotation speed (decreased maximum speed) of the pickup roller, a carrying speed of the sheet slows down because of a continuing state in which a rotation speed of the sheet is decreased by the pickup roller. Therefore, the requirement of high speed processing is not satisfied in the image forming apparatus.